On Wednesday the heads of four United Nations aid organizations joined the European Union in calling the Syrian civil war "the greatest humanitarian tragedy of our times." But that tragedy threatens to be just the beginning of a downward spiral throughout the Middle East.

The U.N. officials urged both sides in the Syrian conflict to call a humanitarian cease-fire so that aid can be delivered to hundreds of thousands of civilians who have been cut off from food and other necessities, largely by the government of Bashar al-Assad.

The head of the World Food Program operation in Syria, Matthew Hollingworth, said, "If the conflict continues, the refugee crisis could cause serious instability throughout the region - the outcome could be catastrophic."

That applies to the entire Mideast, where there has been a war or a constant risk of one in each of the last eight decades. Syria has been an open or covert participant in seven of the eight wars that have swept the region since the 1940s - at least five involving attacks on Israel.

Last week, the U.N. said Syria's government had used the poison gas sarin on four occasions after President Barack Obama declared that its use was a "red line" offense that must be punished. But on the same day of that U.N. announcement, the U.S. and Britain suspended aid to Syrian rebels fighting to oust President Assad because their movement has been taken over by Islamist radicals.

The suspension came a day after Islamist forces overran the headquarters of the main pro-Western Syrian opposition group, seizing a warehouse full of useful non-lethal U.S. aid in the culmination of intra-rebel fighting that has allowed President Assad's forces to make progress in conquering rebel-held areas.

This outcome was foreshadowed by President Obama's decision last year to deny the Syrian rebels lethal arms, and his choice this year to accept Assad's offer to surrender his chemical weapons, effectively sidelining the U.S. as a factor in the Syrian civil war. That left the pro-Western rebels with no strong outside backers.

Instead, al-Qaida has regained a major foothold in the Mideast. Al-Qaida's comeback portends future instability in the region, a prospect that will be amplified if Iran succeeds in its desperate efforts to save the Syrian regime.

Meanwhile, though Iran's nuclear ambitions have supposedly been stalled by an agreement it signed last month with an international coalition, including the U.S., it's difficult to invest much confidence in that accord. After all, Iran remains the world's top state sponsor of terrorism - and its ayatollah leadership claims that its nuclear options remain open.

That adds to the rising threat of Mideast chaos, which is a serious menace to not just the region, but the world.